In direct communication systems, signal synchronization is very important for data transmissions because good communication quality can be ensured only on the basis of a correct synchronization status among direct communication apparatuses. Referring to FIG. 1, in a conventional direct communication system 1, a direct communication apparatus 13 located within communication coverage within the base station 11 is synchronized directly with the base station 11, while a direct communication apparatus 15 located outside the communication coverage of the base station 11 may accomplish the synchronization process according to a synchronization signal forwarded by the direct communication apparatus 13 located within the communication coverage of the base station 11.
However, in the current direct communication technologies, an abnormal synchronization status between the direct communication apparatus 13 and the base station 11 (e.g., signal shielding, signal interference, a low power level of the direct communication apparatus 13, or a temporary abnormal status of the synchronization circuit) might cause an error in synchronization of the direct communication apparatus 13. Consequently, reliability of the synchronization process of the direct communication apparatus 15 that is synchronized via the direct communication apparatus 13 will also be affected, and this significantly degrades the synchronization effectiveness of the whole network.
Accordingly, a need exists in the art to overcome the aforesaid shortcoming of the conventional direct communication system so that the synchronization reliability of the direct communication apparatus located outside the communication coverage of the base station can still be maintained when the direct communication apparatus located within the communication coverage of the base station experiences an abnormal synchronization status.